This invention relates to regulating amplifiers and more particularly to improved method and apparatus for broadening the range of input signal levels over which a regulating amplifier provides the desired regulation.
Regulating amplifier circuits are generally described in U.S. Pat. No. 4,145,665 of O. G. Wisotzky and T. L. Blackburn, issued Mar. 20, 1979, Ser. No. 881,464, filed Feb. 27, 1978, and U.S. Pat. No. 3,962,650 of M. J. Gay, issued June 8, 1976. A regulating amplifier circuit generally comprises a control circuit having a fixed resistor, a varistor, and a control transistor electron control means electrically connected in series between a fixed DC bias voltage and a ground reference potential, the junction of the resistor and varistor being connected to an input terminal of a transistor amplifier electron control means. The control transistor is responsive to a control voltage for adjusting the level of current passed by the varistor for changing its resistance and thus the gain of the regulating amplifier for maintaining the level of an output signal relatively constant. As the level of an AC input signal voltage increases, current passed by the control transistor is increased to maintain the AC signal at the input terminal of the transistor amplifier relatively constant for providing the desired regulation. The gain of the regulating amplifier is initially adjusted to provide a minimum control current for low level AC input signals. The maximum level of AC input signals for which the desired regulation is obtained occurs when the voltage across the fixed resistor results in saturation of the control transistor. This problem is identified at column 6, lines 2-5, of the Gay patent. In order to prevent this saturation condition, the DC bias voltage could be raised to allow a higher control current and correspondingly higher input signal levels. It is necessary, however, that the maximum DC bias voltage be maintained less than that for which clipping occurs in the output signal. This limits the maximum input signal level for which the overall circuit operates to provide the desired regulation.